The earliest guitar‑like instruments probably had flat fretboards, but these days pretty much every guitar has a curved ’board. We’re not talking about an up‑bow or back‑bow, but rather the curve that joins both edges of the fretboard with its highest point all along the centre line of the neck.
If you’re wondering why a fretboard radius is generally preferred, try forming an imaginary barre chord with your fretting hand and you’ll probably see that your index finger is slightly curved. Now grab your guitar and try barring across all six strings using your index finger alone and you may notice that the D and G strings are the ones least likely to play cleanly.
Going back to our ‘air’ barre chord, place your thumb where the back of the neck would normally be and your index finger will probably straighten instinctively. The thumb’s bolstering effect and the fretboard radius conforming to the natural curve of the index finger combine to make chord playing more
